This invention relates generally to two-way cable systems carrying digital signals using cable modems, as well as carrying video broadcast signals, and more specifically this invention relates to blocking and isolating upstream ingress noise on cable systems.
Two-way cable modems provide the digital signal interface between digital computing devices and cable systems, typical coaxial systems carry an array of analog and digital signals according to a predefined frequency plan. As such, cable modems operate in a hostile signal environment wherein the upstream cable channels are subject to varying levels of uncontrollable ingress-noise. While careful plant maintenance is needed to minimize the error causing effects of such noise, there are nevertheless many older cable systems that are considered unacceptable candidates for reliable two-way transmission. The cumulative noise level and the lack of an easy way to determine the source of such noise and to correct its cause substantially reduce the viability and value of such systems for two-way digital communications.
In the downstream direction, cable systems are similar in operation to a water distribution system. Signals are distributed like water flowing under pressure which is split into a myriad of smaller and smaller pipes to reach each house. In the reverse direction, upstream, cable systems are analogous in operation to a sewage system. Signals and noise, like effluent and sewage from each hook-up, contributes to a common flow gathered together by the system to feed a common point. In the case of a cable system, it is the headend, where it is desirable to extract signals from noise. Electrical noise pollution from any single house, hookup or ingress point can thus poison the common upstream channel for all subscribers in that accumulating path. Once polluted, discovering the source of that pollution has been a highly manual-labor intensive effort.
Seventy-five percent or more of the noise which impinges on an upstream channel is ingress noise typically generated within the home hookup. The cable plant operator has little or no control over such sources of spurious electrical interference. An estimated fifteen to twenty percent of the noise is attributable to the typically poor wiring and poor isolation provided by the common low-cost, flexible drop cables using inexpensive F-connectors used to connect the hookups to taps on the solid aluminum sheathed feeder cables passing the houses parallel to the street, also giving rise to ingress noise from adjacent sources. Only a small percentage of the noise is due to ingress into the feeder cable portion of the cable system. Such noise is relatively easy to control as part of regularly-required field maintenance. (A technician can detect leaks by driving along the feeder cable and listening on a monitor to a cable frequency. Egress and ingress are two manifestations of the same phenomenon and are proportional to one another.)
The causes of the noise within a subscriber's premises can be as varied as do-it-yourself cable wiring, light dimmers, universal electric motors with sparking brushes, poor electrical power wiring and inadequate grounding. The obvious problem of access to a subscriber's premises is compounded further in that the noise is often intermittent.
A heavy price is already being paid by the cable industry to address limitations of the upstream channel. For example, the cable industry has tentatively chosen to use QPSK modulation in the upstream direction for two-way data communication. QPSK modulation has only about 1/3 of the data carrying capacity as 64-QAM modulation, which is the preferred standard downstream modulation.
Another high price being paid is the necessity of reducing the number of houses connected to a common cable in a preferred hybrid fiber coax (HFC) system configuration. To reduce the number of households on any coaxial cable feed, fiber optic cables must be extended further into the cable plant to join the coaxial tails to correspondingly reduce the number of cable line amplifiers and houses connected on any single upstream channel and thus reduce the potential sources of ingress noise. For example, when the number of houses per upstream channel is reduced to 500 from 5,000, the probability of noise from any single source destroying the common channel is reduced by a factor of ten. While an excellent partial solution, noise from a subscriber remains a significant risk factor.
So-called legacy cable systems are existing systems, some of which have been designed to use the spectrum below TV Channel 2 (i.e., 5-42 MHz), for upstream transmission. Experience has shown this to have been be a poor standard for transmission because of the frequency-specific ingress noise problem from sources in the ambient environment. What is needed is a viable approach to minimize ingress of noise to allow the effective use of this band for upstream transmission. The specific problem addressed by the invention is the better commercial utilization of the 5-42 MHz band (particularly the 5 to 20 MHz portion of that band), so prone to short-wave radio signal pickup, power line transients, harmonics and impulse noise.